1. Field of the Invention
The present invention relates to an electronic component, an illuminating device in which the electronic component is incorporated as a light emitting unit, and a contact-type image sensor or an image reading device in which the illuminating device is incorporated.
2. Description of the Related Art
Any image reading device, such as a facsimile, a copier, and an image scanner, includes a line illuminating device that linearly illuminates the surface of a source document across a main scanning range. The line illuminating device is configured in such a way that a light emitting unit is disposed on at least one end (one end or both ends) of a rod-shaped or plate-shaped transparent light guiding member and the light incident on the end of the light guiding member exits through an exit surface provided along the longitudinal direction while being repeatedly reflected off inner surfaces of the light guiding member.
In recent years, there has been a need to increase the speed at which an image is read. To this end, it is necessary to increase the luminance of the illuminating device and increase the luminance of the illumination light that illuminates the surface of a source document to be read accordingly. However, when the current conducting through light emitting elements is increased in order to increase the luminance of the illuminating device, the resultant enhanced light emission is accompanied by increase in junction temperature (the light emitting elements themselves generate heat).
In general, the light conversion efficiency of a light emitting unit mounted on an illuminating device depends on the temperature of the atmosphere to which light emitters are exposed. The efficiency lowers as the temperature of the atmosphere rises, and the resistance of the light emitting unit lowers with the temperature. The magnitude of the current thus increases when constant-voltage driving is employed. To avoid such a situation, constant-current driving is typically employed to stabilize the luminance. In consideration of the Arrhenius scaling law (when the temperature decreases by 10degrees, the lifetime doubles), it is known that lowering the temperature of the light emitting unit extends the lifetime thereof.
In particular, to read an image at a higher speed than that typically used in an image reading device, it is conceivable to increase the magnitude of the current flowing through an LED chip to increase the brightness of the illuminating device. Since an LED chip is a semiconductor device, nonradiative recombination more likely occurs at higher temperatures, which lowers the light emission efficiency. It is therefore necessary to appropriately dissipate heat generated in the LED chip into the atmosphere and prevent the temperature of the LED chip from excessively increasing.
To dissipate heat, Japanese Patent Laid-Open No. 2005-217644 proposes a structure in which a plate-shaped common electrode has an extension that serves as a heat dissipater. When the area of the heat dissipater is increased particularly to enhance the heat dissipation efficiency, the larger heat dissipater may interfere with other components. To address the problem, Japanese Patent Laid-Open No. 2005-217644 discloses a configuration in which the heat dissipater is folded along a case in which a transparent light guiding member is housed.
The structure of a light emitting unit of related art, including the light emitting unit described in Japanese Patent Laid-Open No. 2005-217644, includes a metallic substrate 100, an insulating layer 101 formed on the surface thereof, a circuit pattern 102 formed on the insulating layer 101, a solder resist 103 applied onto the portion other than the circuit pattern 102, and an LED chip 104 attached to the circuit pattern 102 by using solder 105, as shown in FIG. 5.
To dissipate the heat generated in the LED chip 104 through a heat dissipating plate in the structure of related art, the heat is conducted to the heat dissipating plate through the substrate 100. The insulating layer 101 is, however, interposed between the substrate 100 and the LED chip 104. The heat conductivity of the insulating layer 101 is approximately 3 W/m×K, which is at least 10 times lower than the heat conductivity of terminals (Cu) (approximately 389 W/m×K). An adequate heat dissipation effect cannot therefore be expected by the heat dissipating structure of related art.